Engine systems may utilize recirculation of exhaust gas from an engine exhaust system to an engine intake system (intake passage), a process referred to as exhaust gas recirculation (EGR), to reduce regulated emissions. An EGR system may include various sensors to measure and/or control the EGR. As one example, the EGR system may include an intake gas constituent sensor, such as an oxygen sensor, which may be employed during non-EGR conditions to determine the oxygen content of fresh intake air. During EGR conditions, the sensor may be used to infer EGR based on a change in oxygen concentration due to addition of EGR as a diluent. One example of such an intake oxygen sensor is shown by Matsubara et al. in U.S. Pat. No. 6,742,379. The EGR system may additionally or optionally include an exhaust gas oxygen sensor coupled to the exhaust manifold for estimating a combustion air-fuel ratio.
As such, due to the location of the oxygen sensor downstream of a charge air cooler in the high pressure air induction system, the sensor may be sensitive to the presence of fuel vapor and other reductants and oxidants such as oil mist. For example, during boosted engine operation, purge air may be received at a compressor inlet location. Hydrocarbons ingested from purge air, positive crankcase ventilation (PCV) and/or rich EGR can consume oxygen on the sensor catalytic surface and reduce the oxygen concentration detected by the sensor. In some cases, the reductants may also react with the sensing element of the oxygen sensor. The reduction in oxygen at the sensor may be incorrectly interpreted as a diluent when using the change in oxygen to estimate EGR. Thus, the sensor measurements may be confounded by the various sensitivities, and the accuracy of the sensor, and thus, measurement and/or control of EGR, may be reduced.
In one example, some of the above issues may be addressed by a method for an engine comprising: in response to ingestion of purge or crankcase hydrocarbons during EGR flow, increasing a reference voltage applied to an intake manifold oxygen sensor; and adjusting EGR flow to the engine based on an output of the sensor at the increased reference voltage. In this way, the hydrocarbon effect on the sensor can be nullified and the accuracy of EGR estimation can be improved.
For example, during EGR conditions when purging and/or positive crankcase ventilation (PCV) is not enabled, a lower (nominal) reference voltage may be applied to the intake manifold oxygen sensor and EGR may be estimated based on a pumping current output by the sensor upon applying the nominal voltage. By comparing a change in the sensor output upon applying the lower voltage relative to a reference point indicative of sensor output during no EGR, the corresponding change in oxygen concentration can be used to infer the EGR dilution of the intake aircharge. An EGR flow can then be adjusted based on the estimated EGR relative to a desired EGR flow. In comparison, during EGR conditions when purging and/or PCV is enabled, a higher reference voltage may be applied to the intake manifold oxygen sensor and EGR may be estimated based on a pumping current output by the sensor upon applying the higher voltage. For example, the nominal reference voltage may be 450 mV while the higher voltage may be at or above 800 mV. At the higher voltage, the intake oxygen sensor goes from reacting hydrocarbons with oxygen at the sensor to dissociating the products of the reaction (namely, water and carbon dioxide). The sensor output at the higher voltage therefore reflects the oxygen concentration due to the EGR dilution effect only and not the oxygen reduction due to reaction of the ingested hydrocarbons on the sensing element. By comparing a change in the sensor output upon applying the higher voltage relative to a reference point indicative of sensor output during no EGR, the corresponding change in oxygen concentration can be used to estimate the EGR dilution of the intake aircharge and accurately adjust an EGR flow.
In this way, by applying a higher reference voltage to an intake manifold oxygen sensor during purging and crankcase ventilation conditions, the effect of the ingested hydrocarbons on the output of the sensor can be nullified. As such, this reduces the corruption of the sensor output in the presence of purge air or blow-by gas hydrocarbons. By improving the accuracy of EGR dilution estimation in the presence of purge air or crankcase gases, EGR control can be improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.